As restrictions on the emission of acid rain precursors such as sulfur dioxide have grown in recent years, a corresponding need has arisen for technologies capable of removing such compounds from gaseous streams in which they are present. Examples of such gaseous streams include flue gases from incinerators and fossil-fueled power plants. It is well known to use activated carbons and cokes for the removal of sulfur dioxide from such streams, especially above 100.degree. C. Below 100.degree. C. and in the presence of oxygen and water, carbonaceous chars can act as catalysts for the oxidation of sulfur dioxide to sulfuric acid. However, the rate of reaction is usually so low that the loadings and concentrations of sulfuric acid which accumulate on the char are rarely of commercial interest.
The use of metals supported on carbonaceous chars has been found to improve the rate of oxidation of SOx. However, such chars containing impregnated metals have inherent disadvantages which may limit their use. For example, the metals may be leached from the carbon surface, contaminating the recovered sulfuric acid. As metal is lost from the carbon surface, the carbon-supported catalyst may also become less effective. Finally, at the end of its useful life, the spent metal-impregnated carbon may become an environmental and disposal problem because of the impregnated metals, many of which are environmentally hazardous. Given these potential liabilities, the use of a carbonaceous char which does not contain impregnated metals would be preferred if a char with suitable inherent catalytic activity were available.
The use of high-temperature carbonaceous chars containing no impregnated metals and treated with nitrogen-containing compounds or prepared from nitrogen-rich starting materials have shown particular promise for SOx removal. For example, improvements in the rate of oxidation of sulfur (IV) to sulfur (VI) have been achieved in the liquid phase using high-temperature, nitrogen-poor carbonaceous chars which have been heated at temperatures above about 700.degree. C. in the presence of ammonia or other nitrogen-containing substances. Improved direct gas-phase oxidation of sulfur dioxide has been achieved for high-temperature nitrogen-poor chars contacted with melamine or its derivatives at temperatures above about 700.degree. C. Other carbonaceous chars have been used for the enhanced liquid-phase and gas-phase oxidation of sulfur (IV). These carbonaceous chars were derived from nitrogen-rich starting materials such as polyacrylonitrile. Also, a char suitable for the catalytic oxidation of sulfur (IV) has been produced wherein a high-temperature nitrogen-poor activated carbon or coke was oxidized by sulfur (VI) and exposed to a nitrogen-containing ammonia salt at temperatures above 350.degree. C.
The prior art methods for improving the inherent catalytic capabilities of carbonaceous materials for sulfur (IV) oxidation have certain disadvantages which limit their overall utility. For example, most of these teachings utilize high-temperature chars as starting materials. High-temperature carbonaceous chars are those produced by thermal treatment at temperatures equal to or greater than 700.degree. C. Low-temperature chars have not experienced temperatures greater than 700.degree. C. Since high-temperature carbonaceous chars are fairly inert chemically, the use of aggressive chemical post-treatments is usually required to effect significant changes in their catalytic capabilities. One exception is the use of an expensive synthetic starting material such as polyacrylonitrile. However, this is also disadvantageous due to the high costs of such materials. The use of high-temperature finished chars as starting materials also is inevitably more expensive than the direct use of the raw materials from which they are made. In some processes for the production of these chars, large quantities of toxic process byproducts such as sulfur dioxide and cyanide are unavoidably produced, while others require the use of highly hazardous treatment agents such as sulfuric acid.
Accordingly, it is the object of the present invention to provide an improved process for the removal of sulfur oxides from gas or vapor media containing oxygen and H.sub.2 O using a carbonaceous char catalyst prepared directly from a nitrogen-poor, naturally-occurring starting material such as a bituminous coal or a bituminous material. It is also the object of the present invention to provide significant removal of sulfur oxides by the use of a carbonaceous char which contains no impregnated metals. It is further the object of the present invention to limit the use of agents responsible for imparting catalytic activity to the carbonaceous char by performing the essential treatments at low temperatures (less than 700.degree. C.) and during transition of the starting material to the final product. These treatments include low-temperature (less than 700.degree. C.) carbonization and oxidation of the starting material to produce a low-temperature oxidized char, preferably by inexpensive, abundant, relatively non-toxic oxidants such as air, and contacting of the oxidized low-temperature char with one or more nitrogen-containing compounds prior to, not after, the initial exposure of the char to high temperatures (greater than or equal to 700.degree. C.). The preferred nitrogen-containing compounds are inexpensive, abundant, and relatively non-toxic agents such as urea or melamine, although other nitrogen-containing compounds may also be used including ammonia and derivatives of urea, melamine and ammonia. By this method, carbonaceous chars with high catalytic activity, per se, for the oxidation of sulfur (IV) in the presence of oxygen and H.sub.2 O can be produced with minimal departure from conventional processes for the production of high-temperature carbonaceous chars such as activated carbons and cokes.